The present invention relates to communication systems and more particularly to a system for the conversion of analog signals to digital for digital storage and for the retrieval and reconversion to analog format of such signals for playback.
The prior art is replete with various types of paging systems and radio operated systems by which a message can be left for an individual who is not able to be contacted directly for one reason or another. For example, many paging systems operate with a large complex central processing facility in which messages are queued and transmitted, normally in digital form, to subscribers along with the subscriber""s address code. The subscriber to the service carries a paging unit which is pre-programmed to activate upon receiving a message which is preceded by the address code for that paging unit. The pager then normally emits an audible sound to alert the subscriber that a message is being received and stored for him. The message is placed in the pager memory and the messages is retrieved in the form of a display message, normally on an LED or LCD display screen. Although such systems are efficient and require very little air time in order to send the digital message, the messages transmitted are necessarily of limited duration and are normally of the type which require the subscriber to go to the nearest telephone and call the message originator. In addition, unless the transmitted messages are strictly numeric, i.e. telephone numbers and the like, alpha numeric messages require a special terminal in order to input the alpha numeric message to be transmitted and paging systems of this type require expensive computerized central message facilities.
Other paging systems are available which utilize a transmitter which transmits in analog form an audio message preceded by an address code which is received by a pre-programmed receiver. The message is played immediately upon receipt and in some units the message can be recorded on a tape cassette for replay. Pagers of this type are normally relatively bulky and require substantially high power requirements to drive the mechanical portions of the tape recorder.
In the area of telephone communications, answering machines are available which are provided with one or more tape cassettes for playing a message to the caller to indicate that the called party is not available to answer the phone and to record a message for later playback. Although answering machines are readily available for single line use and their price is becoming more and more reasonable, such devices are normally not available as part of the telephone circuitry itself and most of the existing answering machines are bulky and require a substantial amount of desk top space. Furthermore, answering machines are not readily available for multi-line business phones and com-line recording.
Yet another form of message service is the so called voice storage retrieval system (VMS) in which a voice message can be left at a central message storage facility and the subscriber, by use of a specific code, can access the memory at the central computer to retrieve the message. These systems are expensive to operate in view of the necessity of powerful computers at the central system facility to process and store the messages and in addition can be inconvenient to use since the subscriber must find a telephone in order to receive the message. In addition, messages may not be timely received because the addressee inadvertently fails to check for messages.
In the area of two-way radio communications, such as in the case of police and fire communications, emergency communications and the like where the addressee may be away from the mobile unit from time to time, many systems employ the use of hand held receivers, i.e. walkie-talkies, which may be patched into the mobile receiver for the receipt of incoming messages while the operator is away from the unit. Such devices are expensive and in many cases would be totally unnecessary if a reliable, inexpensive message storage system were available at the mobile unit. Some systems are available which are similar to the telephone answering machines for transmitting a prerecorded message and for recording incoming messages when the operator is not at the mobile unit. These systems have been found to be bulky, unreliable and inflexible in connection with radio communications. A more sophisticated system has been promulgated in U.S. Pat. No. 4,468,813 Burke et al, and in U.S. Pat. No. 4,495,647 Burke et al. This system requires a base unit which sends a command program packet in digital form to the mobile unit which is programmed to respond to the command program for receiving the message in analog form. Responsive to the command program, the mobile unit converts the message to digital form for storage and responsive to a termination command set in digital form by the base unit, the mobile unit recording system is deactivated. The operator at the mobile unit can then replay the digital message in analog form. The system as disclosed in the aforementioned U.S. Patents requires a sophisticated encoding system at the base transmitter which is capable of generating a command program packet and the termination code signal. Furthermore, the base transmitter must be capable of transmitting the command packet in the form described in the aforementioned patents. The mobile unit must be capable of receiving and decoding the command program packet and transmitting its own command program packet back to the base unit. The mobile unit utilized in such a system requires two separate power supplied which would render the device unsuitable for portable hand held receivers such as pagers and the like.
In accordance with the present invention there is provided an analog to digital data storage system which is readily adapted for use in communication systems such as paging systems, telephones, multi-line telephones, cellular telephones, intercom and telemetry systems, two-way radios, and the like, by which analog signals including voice messages and data transmission can be received, converted to digital format and stored in memory in digital format for retrieval, reconversion to analog form and playback as desired. The data storage system is easily installed on existing telephone and radio equipment at low cost and operates with very low power requirements. The data storage system is adapted to be activated by any conventional analog or digital address encoder such as, for example, digital code, tone, dual tone multiple frequency (DTMF) or may even be voice actuated (VOX). No specially modified transmitter is required for use with the system of the present invention. In addition the means for deactivating the circuitry after receipt of a message is contained within the data storage system itself and except for an address code (which is preferred but not critical), the necessity of transmitting a packet of command data to control the reception and recording of the message at the receiving unit and a termination code at the end of a message to deactivate the receiving unit is eliminated.
In accordance with the present invention the analog to digital data storage system includes receiver means for receiving an incoming signal carrying an analog component from a suitable transmitter. The analog signal is preferably preceded by a designated address code which is specific to a particular data memory system. The system further includes enable means activated by the incoming signal to emit an enable signal (logic high) to activate the system circuitry as will be hereinafter described and illustrated. The enable means may comprise a decoder which has been programmed to recognize an address code specified to a particular receiver or group of receivers. The enable means may also be activated by the incoming signal without an address code, such as in the case of a VOX circuit, to emit the enable signal. The system also includes conversion means for converting the incoming analog data to digital format and memory means for storing the converted digital data. The conversion means further includes circuitry for reconverting a digital signal to analog format. Control means are provided for activating the conversion means and the digital memory storage means responsive to the enable signal from the decoder means. In the preferred embodiment, the control means also acts to deactivate the conversion means and memory means at the completion of the message or after a predetermined period of time. Switching means are included for activating the conversion means and the memory means for playback in analog format of any stored messages. The system further includes amplifier means for listening to incoming analog signals and playback of stored messages.
The system of the present invention is readily adaptable for use with wire communication systems such as single and multi-line telephone systems, intercom systems and for radio communication. Thus the system of the present invention is useful for paging systems, two-way radio, cellular telephones, conventional telephone intercom systems, and telemetry systems. In a preferred form of the invention the system is adapted for receiving analog messages which are transmitted at high speed and for playback of such messages after retrieval from memory at a slow speed so that the message may be understood by the mobile operator. In this fashion, air transmission time is substantially reduced which is of critical importance in those areas where the radio frequencies are crowded, such as in paging systems where assigned frequencies are limited and there are a large number of subscribers utilizing the system.